The objectives of this project are to elucidate the nature of magnetic spin-spin interactions between two paramagnetic species formed in homolytic cobalt-carbon bond cleavage of coenzyme B12 and related model compounds in a polycrystalline state or in model membranes induced by light. Experimental studies are planned using multi-frequency (Q-band, X-band, S-band and L-band) electron spin resonance (ESR) spectroscopy on polycrystalline coenzyme B12 and model compounds with known structures based on X-ray determinations. Similar studies are to be performed on coenzyme B12 and with model compounds dispersed in two different lipid bilayers: L-alpha-dimyristoylphosphatidylcholine a saturated lipid and egg yolk phosphatidylcholine an unsaturated lipid. With regard to the ESR measurements, the lineshapes will be calculated using either matrix diagonalization techniques or perturbation methods. The values of the cobalt-radical spin-spin coupling constants, J0, will be determined from both lineshape simulation and the temperature dependence of the area of the ESR signals. The dipolar distance between the two spins in the triplet state will be estimated using both lineshape simulation and a general relationship comparing the relative intensity of the half-field line vs. the full-field line. The relative positions of the Co(II) and the free radical can then be determined from the "magnetic geometry" idea which is well defined by multi-frequency ESR lineshapes. In addition, we intend to determine parameters delta eta not equal to, delta S is not equal to and the bond dissociation energies of the cobalt- carbon bond of coenzyme B12 and the model compounds from the temperature dependence of the rate of homolysis by photolytic cleavage in model membranes. The participation or the contribution of the axial ligand benzimidazole in the activation and the rate of homolysis is of interest which will be determined using both base-on and base-off coenzyme B12 model compounds. These studies may lead to a better understanding of coenzyme B12 in biochemical systems.